The conversion of solar energy into electrical energy is becoming more important as costs and shortages of more standard energy sources continue to increase. Standard solar cells made from inorganic semiconductors, such as silicon cells, have been studied since the 1950s and have been used as renewable electric power sources for generating power in a wide variety of applications ranging from orbiting satellites to solar farms to portable devices. While relatively efficient, inorganic solar cells often have high dollar and environmental costs associated with manufacturing as the cells are made of toxic materials that typically have to be grown as a single large crystal.
More recently, solar cells utilizing organic materials have appeared, which has raised the intriguing possibility of solar cells that are relatively inexpensive and can be fabricated on flexible substrates. It is envisioned that organic solar cells could be fabricated or even painted on large flexible substrates, which would allow the created cells to be folded or rolled up in a briefcase or backpack, to be worn, or to be deployed to provide electricity for wearable electronics, for recharging batteries, and for low-power devices. Unfortunately, the solar conversion efficiencies of existing organic solar or photovoltaic (PV) cells is typically less than 10 to 20 percent of the solar conversion efficiencies obtainable in inorganic PV cells or in some cases, only 1 to 2 percent conversion efficiency. There is a need to find a method of producing organic PV cells that are more efficient in converting solar energy to electricity and that still provide the advantages of working with organic materials, i.e., being inexpensive and relatively easy to manufacture in high volumes on flexible substrates. Once such a manufacturing method is found, flexible organic solar cells likely will be widely used to provide efficient distributed power to homes and buildings and to provide power in applications requiring portability, such as many military and consumer uses.
In developing an improved technique of manufacturing organic solar cells, it is important to understand the differences between organic solar cells and the more common inorganic solar cells and to understand manufacturing problems associated with organic solar cells. A key difference between organic photovoltaic (OPV) cells and conventional inorganic photovoltaic (IPV) cells, e.g., the silicon p-n junction cell, is the relative importance of interfacial processes between layers of semiconductor materials (such as between p-type and n-type semiconductor layers). This difference is closely related to the charge generation mechanism needed for producing a useful PV cell. In IPV cells, electron-hole pairs are generated immediately upon light absorption throughout the bulk of the material according to the exponential decrease of the incident light intensity. Since the electrons and holes are distributed spatially within the same material, the photoinduced chemical potential energy gradient drives both the electrons and holes in the same direction and recombination of the holes and electrons needs to be controlled throughout the bulk.
In contrast, light absorption in OPV cells typically results in the production of a mobile excited state, i.e., a tightly bound electron-hole pair often called an exciton, rather than a free electron-hole pair. Dissociation of the excitons occurs at the heterointerface between two dissimilar organic semiconductor materials or layers. Hence, light absorption results in free electrons in one of the two semiconductor materials and free holes on the other side of the interface in the other semiconductor material, with the free electrons and holes being driven in opposite directions away from the interface. In other words, carrier generation is simultaneous to, and identical with, carrier separation across the interface in OPV cells. Recombination of the free electrons with the holes is a bane of IPV and OPV cells. However, due to the differences in the dissociation mechanisms of OPV and IPV cells, recombination is a particular problem at the interface in OPV cells. Recombination is a larger problem at the interface in OPV cells due to the much larger concentration of carriers at the interface when compared to IPV cells in which the carriers are distributed in the bulk.
In practice, a uniform electric field is typically applied across the PV cell to cause the electrons to separate from the paired holes. This is an effective approach for IPV cells in which the electrons can only be separated from the holes by applying such an electrical potential-energy gradient, but application of a uniform electric field across the bulk has not proven effective in OPV cells where a critical efficiency limitation is recombination at the interface of the two semiconductor layers. More particularly, under “forward” bias, the OPV cell typically will only produce small current levels as a consequence of this interfacial recombination and of electrons being photogenerated on one side of an interface and holes on the other. When the applied bias drives the photogenerated carriers back toward the interface (i.e., forward bias for a p-n junction), the carriers typically recombine and only a small number is thermally emitted over the heterointerface energy barrier (i.e., an energy band diagram shows an energy offset at the interface).
To enhance efficiency including addressing a low forward bias current, IPV cells rely on the ability to dope semiconductors precisely both in magnitude and spatial extent. Unfortunately, doping of organic materials is a difficult and often impractical process that has not addressed the low efficiency issues in OPV cells. One difficulty with doping in OPV cells is that the integrity of the organic semiconductor materials are often chemically or morphologically disturbed, which can create exciton traps or quenching cites that block diffusion and/or cause recombination.
Crystalline organic semiconductors have been shown to have superior optical and electrical characteristics when compared to amorphous films of the same materials. However, single crystals of organic semiconductors generally cannot be used for devices, such as for OPV cells, because the crystals are quite small, i.e., 0.1 to 10 μm, and do not adhere well to most substrates or to other organic semiconductors. This lack of good adhesion is a problem that must be addressed in manufacturing an OPV cell because good crystal-to-crystal contact is required throughout the cell to maintain electron and hole paths. When the conventional method of sublimation is used to apply organics to a surface, the resulting film is commonly amorphous or nanocrystalline but is usually not strongly adherent and often causes serious interfacial adhesion problems. Attempts have been made to address this problem with annealing by heating or exposing the films to a solvent vapor, but to date, the resulting crystalline films have had numerous pinholes that have led to semiconductor devices with unacceptable electrical shorts.
Hence, there remains a need for improved organic photovoltaic cells, and methods of manufacturing such cells, that provide enhanced solar conversion efficiency while facilitating the manufacture of high efficiency, inexpensive, and flexible organic solar cells. Preferably, such cells will address the problem of recombination at the heterojunction interface while utilizing readily available and inexpensive materials that can be processed utilizing relatively well-known manufacturing techniques.